<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>border-box/ content-box</title>
  <style>
      div {
          width: 200px;
          height: 100px;
          padding: 10px;
          border: 5px solid black;
          margin: 20px;
      }

      .left {
          background: red;
          box-sizing: border-box;
      }

      .right {
          background: #ccc;
          box-sizing: content-box;
      }
  </style>
</head>
<body>
<div class="left">border-box</div>
<div class="right">content-box</div>
</body>
</html>
